Photochemistry of radical ions with irradiation of electron and laser pulses

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Photophysical and chemical properties of radical ions of aromatic compounds in the excited states have been clarified by a pulse radiolysis-laser flash photolysis combined method. The lifetimes of radical cations and anions of cis- and trans-stibene (c-St and t-St) and radical anions of dicyanoanthracene, phenazine, and anthraquinones in the excited states were measured for the first time and their transient phenomena such as the photoelectron ejection, c-t isomerization, and selective hole and electron transfer quenching of the radical cations and anions in the excited states, respectively, were investigated. In the deactivation of radical cations of c-St in the excited doublet state (c-St^<*+**>), the internal conversion competes with the isomerization to t-St^<*+> in a quantum yield of 0.49, whereas t-St^<*+**> is only deactivated via internal conversion. Lifetimes of t-St^<*+**> and c-St^<*+**> are estimated to be approximately 240 and 120 ps, respectively. The shorter lifetime of … More c-St^<*+**> is attributed to isomerization and conversion to another product via twisting about the central C=C double bond. St radical anions in the excited doublet state (St^<*-**>) undergo unimolecular isomerization from c-St^<*-> to t-St^<*->, photoelectron ejection, and electron transfer to biphenyl added as a quencher. From the selective electron transfer from St^<*-**> to biphenyl (Bp), the lifetimes of c-St^<*-**> and t-St^<*-**> are estimated to be approximately 1.5 and 2.5 ns, respectively. The shorter tau of c-St^<*-**> is attributed to isomerization via twisting about the central C=C double bond. The tau values of c-St^<*-**> and t-St^<*-**> are 1 order of magnitude longer than those of c-St^<*+**> and t-St^<*+**>. Lifetimes of approximately 4 ns are estimated for M^<*-**> (M=dicyanoanthracene, phenazine, and anthraquinones). The shorter lifetimes of 1.0-1.4 ns for methyl and chloro substituents on anthraquinone are explained in terms of internal conversion accelerated by rotatio of the substituents. The energy gap between the excited and ground states of M^<・-> is a singnficant factor for the rate of the internal conversion. A solvated pair of M and quencher^<・-> is suggested as an intermediate in the electron-transfer quenching of M^<*-**> by quencher such as fumaronitile. Less